For instance, a lambda closed-loop control in conjunction with a catalytic converter is currently the most effective exhaust-gas treatment method for the spark-ignition engine. Only in interaction with currently available ignition and injection systems is it possible to achieve very low exhaust values. Limit values for the engine exhaust gas are even mandated by law in most countries.
The use of a three-way catalytic converter, or selective catalytic converter, is especially effective. This type of catalytic converter is able to break down up to more than 98% of hydrocarbons, carbon monoxides and nitrogen oxides provided the engine is operated within a range of approximately 1% around the stoichiometric air-fuel ratio at lambda=1. In this context, lambda specifies the degree to which the actually present air-fuel mixture deviates from the lambda=1 value, which corresponds to a mass ratio of 14.7 kg air to 1 kg of gasoline that is theoretically required for complete combustion, i.e., lambda is the quotient of the supplied air mass and the theoretical air requirement.
As a general principle, lambda closed-loop control measures the particular exhaust gas, the supplied fuel quantity being immediately corrected according to the measuring result via the injection system, fox instance. Used as measuring probe is a lambda sensor, which is able to measure a steady lambda signal around lambda=1 and in this way supplies a signal that indicates whether the mixture is richer or leaner than lambda=1.
As may be known, the effect of these lambda sensors is based on the principle of a galvanic oxygen concentration cell having a solid state electrolyte.
Furthermore, a cylinder-individual lambda closed-loop control may be used to improve the exhaust gas if the lambda sensor, owing to its dynamic properties, is able to track lambda fluctuations in the exhaust-gas flow caused by cylinder-individual lambda differences at the installation location of the sensor.
Due to a temporally high-resolution evaluation of the signal coming from the lambda sensor, it is possible to conclude from the composite lambda signal to the lambda of the individual engine cylinders whose exhaust gas is conveyed to the installation location of the sensor. In this way, cylinder-individual lambda differences may be corrected and the exhaust-gas result or, at the very least, the exhaust-gas stability be improved.
The dynamic characteristics of a lambda sensor in new condition is in most cases adequate within a selected operating range. Nevertheless, in the event that the dynamic characteristics of the sensor change, to the effect that cylinder-individual lambda values are unable to be resolved since the response times of the sensor are increasing, the closed-loop lambda control will not intervene although lambda fluctuations are indeed present in the exhaust gas. Causes of a reduced dynamic performance of the sensor are, for instance, constrictions in the protective tube orifices of the sensor or contamination of function-controlling sensor ceramic parts of the solid state electrolyte as a result of deposits. In broad-band sensors, contamination of the diffusion barrier provided there may also play a part. In the worst case, a non-functioning cylinder-individual lambda closed-loop control will result in non-compliance with the mentioned exhaust-gas limit values mandated by law. In this case, the changed dynamic characteristics of the lambda sensor must be indicated by a control light, for example.